1. Field of the Invention
The present invention relates generally to a system for providing optical energy transmissions and, more specifically to such a system that increases the resolving power, spectral shaping benefits, and efficient transfer of optical power for such transmission by active and precise phase and amplitude control.
2. Description of the Prior Art
Many applications of optical transmission systems, such as communication applications and laser weapon applications, require precise control of a wavefront profile of transmitted optical energy. Aberrations in the wavefront profile typically include phase, focus, or similar astigmatic characteristics that, absent correction, may significantly impact the functional capabilities of the particular transmission system.
Optical communication can be accomplished by transmitting signals on optical carriers and routing these signals based on optical wavelengthxe2x80x94a technique known in the art as wavelength division multiplexing (WDM). Using wavelength division multiplexing, multiple optical signals can be simultaneously transmitted within an optical spectrum by assigning each optical signal a unique wavelength. An important aspect of an optical communication system using WDM, and other similar optical communication methods, is an optical array or optical grating device that has the capability to accurately distinguish each optical wavelength from each other optical wavelength contained within the optical spectrum.
The ability to distinguish adjacent wavelength components is known as the xe2x80x9cresolving powerxe2x80x9d of the optical grating device, and is equal to the ratio between the wavelength at which the device is operating and the smallest change of wavelength that the device can distinguish. In other words, to calculate the resolving power of a particular grating device, it must be determined how close any two wavelength patterns can be without merging into each other.
A key element in an optical grating device""s ability to resolve optical radiation is the use of diffracting elements which form a grating that redirects optical radiation by an amount that is wavelength dependant. In conventional arrayed waveguide grating devices (AWGs), each element of the grating is an optical waveguide of varying length. In conventional fiber optical delay line grating devices (FODLGs), each element is an individual fiber optic delay line of varying length.
Single wavelength radiation arriving incident upon a grating at an angle xcex8i, diffracts at an angle xcex8r according to its wavelength(s) xcex. Specifically, segments of the optical spectrum containing the wavelength(s) xcex are diffracted in order at equal magnitudes along the N grating lines or grooves of the grating. The formation of this xe2x80x9cdiffracted orderxe2x80x9d depends on the wavelength(s) xcex and on the wave segment contributions from each grating line being in phase by an integral number of wavelengths. This means that the path difference or time delay across successive delay lines should be equal to within a whole number of wavelengths, as defined in accordance with the following general grating equation:
incremental time delay=d(sin xcex8ixe2x88x92sin xcex8r)=mxcexxe2x80x83xe2x80x83(1)
where d=distance between any two adjacent grating lines (nm), xcex8i=angle optical radiation incident upon grating (microradians or milliradians), xcex8r=angle optical radiation diffracted by grating (microradians or milliradians), m=diffracted order (integer), and xcex=operating wavelength (nm).
It is evident from equation (1), that if the time delay across successive grating lines or grooves is equal to whole number of wavelengths, the wave segments will be xe2x80x9cin-phasexe2x80x9d (i.e. phase difference equals integer multiples xcex) with each other, and they will add constructively. Proper constructive addition of the wave segments insures that all of the optical energy for the optical spectrum is decomposed through the grating to form a high intensity optical signal that can be spectrally separated by wavelength at the focal plane of the optical grating device. If the wave segments are out of phase by a significant percentage of wavelength, the individual segments will add destructively. Reduced optical power will be emitted from the grating, and the ability to resolve wavelength components at the focal plane of the grating device is reduced.
Several circumstances may contribute to the likelihood of out of phase conditions in optical grating devices. Since conventional optical grating devices are passive devices, the incremental delay of each device is determined by manufacturing tolerances and processes that set the length of each delay line. However, rarely is the required level of precision to within a small fraction of xcex achievable. Additionally, the grating device may later be subject to thermal fluctuations where a temperature variance of as little as fraction of a degree may cause the grating to expand or contract. Such expansion or contraction may cause the delay line path length to change and, therefore, the time delay for one or many grating lines to be out of phase. Similarly, mechanical stress may adversely affect the topology of the delay line and cause out of phase conditions.
The publication xe2x80x9cFabrication of 128-channel arrayed-waveguide grating multiplexer with 25 Ghz channel spacingxe2x80x9d, by Okamoto et al., Electronics Letters Vol. 32 No. 16 pp. 1474-1475, Aug. 1, 1996, is illustrative of the shortcomings of conventional optical grating devices. Specifically, the Okamoto et al. publication discloses achieving a 25 GHz resolution with a 128-channel arrayed waveguide grating (AWG) multiplexer using a planar lightwave circuit (PLC). The Okamoto et al. publication distinguishes its AWG device from other conventional AWG multiplexer devices that are limited to a resolution (channel spacing) greater than 50 GHz. The Okamoto et al. publication describes that at narrower channel spacing, conventional devices xe2x80x9cexhibited higher crosstalk due to phase errors in the array waveguides.xe2x80x9d While AWG""s similar to that described in the Okamoto et al. publication may reduce crosstalk levels and increase resolving power, they may be inadequate in circumstances where manufacturing tolerances, thermal fluctuation and mechanical stress significantly limit the in-phase conditions necessary for analog radio frequency (RF) applications and other applications that require higher levels of signal isolation.
Laser weapon applications are similarly affected by phase, focus and astigmatic aberrations. Such wavefront aberrations are typically the result of vibration and misalignment of the reflecting surfaces, thermal fluctuations that cause warping of the lasing medium and other internal components, and turbulence in the lasing medium. Controlling these wavefront aberrations directly impacts a laser weapon""s ability to determine target ranging, steer the laser beam, and optimally deliver laser power to a remote target.
Thus, an optical transmission system that mitigates the effects of phase misalignment and provides the benefit of spectral shaping by amplitude adjustment is highly desirable.
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention that provides an optical transmission system. The system includes a source for transmitting an input optical signal, an optical array having a primary input for receiving the optical signal and a plurality of delay lines. An optical splitter is located at the primary array input of the optical array and splits the input optical signal into a plurality of delay line signals, where each one of the delay line signals is input to one corresponding delay line and each delay line carries a corresponding delay line signal to a destination point located at the delay line outputs. A means for phase and amplitude modulating the optical wave component of each delay line signal utilizing orthogonal code modulation is provided.